The present invention relates to devices for measuring electrical characteristics and, in particular, to a measurement device having common mode current cancellation.
Voltmeters, current meters, multimeters, source/measure units and other electrical measurement devices are often required to make floating measurements with respect to earth ground on a device under test. If the measurement device is self-powered (e.g., battery operated), little difficulty is encountered in achieving an accurate measurement.
However, in many cases it is necessary or desirable to power the measurement device with line power (e.g., 120 volt, 60 hertz, mains). This line power or line supply is nearly always connected to earth ground (e.g., actual ground or the neutral conductor). Similarly, the device under test is also usually connected in some way to earth ground. Therefore, to provide isolation of the measurement device (as well as for other reasons), a transformer is used between the line supply and the measurement device proper.
For many applications, this isolation is sufficient, but in other applications, the capacitance between the transformer windings is sufficient to allow unacceptable levels of current to flow across the transformer and through the device under test. (The current through the device under test is also typically due to stray capacitances). Shields in the transformer can minimize this common mode current, but only so much (e.g., 2 microamperes at 60 hertz).